1. Field of the Invention
The present invention relates to a developing apparatus for developing an electrostatic latent image formed on an image bearing member with developer.
2. Related Background Art
In the past, it is already known to provide image forming apparatuses in which, after a latent image bearing member is uniformly charged, image exposure is effected by analogue exposure or a semiconductor laser or an LED, and, after an electrostatic latent image formed on the latent image bearing member, the electrostatic latent image is visualized by a developing apparatus as a toner image. After the developer image is transferred to a transfer material as a recording material, the transfer material is separated from the latent image bearing member and is subjected to fixing treatment in a fixing device to obtain a fixed image.
Now, an operation of such a conventional image forming apparatus will be described with reference to FIG. 10.
Such an image forming apparatus includes a photosensitive drum 101 having a photoconductive layer such as OPC or, as a latent image bearing member, a-Si and is rotated in a direction shown by the arrow B.
In such an image forming apparatus, first of all, a surface of the photosensitive drum 101 is uniformly charged by a primary charger 103 to xe2x88x92700 Volts, for example.
Then, the surface potential of the exposed portion on the photosensitive drum 101 is reduced, by image exposure 112 corresponding to image signal information, to xe2x88x92200 Volts, for example, thereby forming a latent image corresponding to the image signal on the photosensitive drum 101. For example, a semiconductor laser or an LED array is used for the image exposure 112.
Then, the latent image is developed by a developing device 102 as a developing apparatus using dry one-component developer to visualize the latent image as a toner image. The developing apparatus using the dry one-component developer has a long endurance life because it is simple and exchange of carrier is not required, and, for example, jumping developing using magnetic one-component toner can be effected. Incidentally, the developing device 102 utilizes negatively charged black toner. Further, upon development, a DC bias of about xe2x88x92500 Volts is applied to a developing sleeve (developer bearing member) of the developing device 102 as a developing bias, thereby reversal-developing the latent image.
Thereafter, if desired, pre-transfer treatment is effected by using a charger 110 (normally, application of corona by DC or AC or photoelectricity removal or combination thereof), and then, the toner image is transferred, by a transfer charger 104, onto a transfer material supplied to the photosensitive drum 101.
Thereafter, the transfer material to which the toner image was transferred is sent to a fixing device (fixing apparatus) 107, where the toner image is fixed to the transfer material as an image. On the other hand, transfer residual toner which was not used in the development and remains on the photosensitive drum 101 is removed by a cleaning device 106, thereby preparing for next image formation.
On the other hand, in order to obtain a high speed image forming apparatus, in the conventional developing apparatus, as disclosed in Japanese Patent Application Laid-Open No. 3-204084 (1991), a plurality of developing sleeves are provided in a developing apparatus using a two-component magnet brush, or, as disclosed in Japanese Patent Laid-Open No. 2-188778 (1990), a replenishing amount of toner from developing sleeves (developer bearing members) to a photosensitive member (latent image bearing member) is made uniform by arranging the developing sleeves and the photosensitive member so the that distance between the developing sleeves and the photosensitive member is gradually decreased toward a downstream side in a rotational direction of the photosensitive member.
Next, a method for sealing developer at an axial end of the rotatable developer bearing member of the conventional developing apparatus will be described with reference to FIGS. 11 to 13.
As shown in FIG. 11, such a developing apparatus has a developer container 102a containing the developer, and agitating means 102B are provided in the developer container 102a so that the developer in the developer container 102a is agitated and conveyed in a predetermined direction by the agitating means 102B.
Further, the developing apparatus has a developing sleeve (developer (magnetic powder) bearing member) 120 for developing the electrostatic latent image formed on the photosensitive drum 101 and for carrying the developer in the developer container 102a toward the photosensitive drum 101.
Normally, the developing sleeve 120 includes non-magnetic metal at its outer periphery and also has a magnet roller (magnetic field generating means) 120a fixed therewithin and having a plurality of circumferential magnetic poles N1, N2, N3, S1, S2, as shown in FIG. 11.
Further, as shown in FIG. 13, a shaft 120c of the developing sleeve 120 is supported by side walls 102e of the developer container 102a via bearings 120b. Incidentally, FIG. 13 shows only one side wall 120e. 
With the above-mentioned arrangement, by the rotation of the developing sleeve 120, the developer is carried from the magnetic pole S3 to the magnetic pole N1 while being borne by the developing sleeve 120, and an amount of developer on the developing sleeve 120 is regulated by a developing blade 102g supported by a support plate 102f to form a thin developer layer on the developing sleeve 120. Incidentally, the magnetic pole N2 is a main developing pole where the electrostatic latent image on the photosensitive drum 101 is developed by the developer. Thereafter, the developer is returned into the developer container 102a by the rotation of the developing sleeve 120.
In the past, in a method for mechanically scraping the developer on the developing sleeve by felt in order to seal the developer at the ends of the developing sleeve, torque for driving the developing sleeve might become great and a great load may act on a motor to shorten endurance life of the motor and the toner may be firmly fixed to the end of the sleeve due to an increase in temperature at the sleeve ends.
Further, the developer circulated within the developer container 102a in the above-mentioned manner is shifted toward the bearings 120b along the surface of the developing sleeve 120 and leaks through the axial ends of the developing sleeve 120 to be scattered in a main body of the image forming apparatus, thereby causing various inconveniences.
In order to prevent the developer from leaking through the sleeve ends, as disclosed in Japanese Patent Application Laid-Open No. 2-262171 (1990), magnet brushes of developer are formed at both axial ends of the developing sleeve 120 along at least a part of the peripheral surface of the developing sleeve 120 with predetermined gaps with respect to the surface of the developing sleeve 120, so that the developer is sealed by the magnet brushes not to leak through the axial ends of the developing sleeve 120 (FIG. 12).
Further, as an improvement, as disclosed in Japanese Patent Laid-Open No. 8-137259 (1996), there has been proposed a method in which sawteeth are provided radially of the developing sleeve to concentrate the magnetic force, thereby sealing the developer not to leak through the axial ends of the developing sleeve.
However, as is in the above-mentioned conventional developing apparatus, in a developing apparatus having a plurality of developer bearing members for high speed operation, although there is an advantage that the developing ability is enhanced, the developer may leak through the axial ends of the developing sleeves (developer bearing members) due to high speed rotations of the developer bearing members.
Further, in the developing apparatus having the plural developing sleeves, the toner scattered in a developing portion at an upstream side in the rotational direction of the photosensitive drum will be dropped onto the developing sleeve disposed at a downstream side in the rotational direction. Particularly, the toner, which is scattered outside of the seal portions provided at the axial ends of the developing sleeve disposed at a downstream side in the rotational direction of the photosensitive drum, may be scattered within the image forming apparatus as the developing sleeve disposed at a downstream side in the rotational direction of the photosensitive drum is rotated. Since such a phenomenon occurs noticeably when alternating electric field is used in the developing portion at the upstream side in the rotational direction of the photosensitive drum or when one-component magnetic developer having no carrier is used, for example, such a phenomenon leads to a serious defect in a one-component jumping developing system. Further, such a phenomenon occurs noticeably in a developing apparatus in which there is no layer thickness regulating means between the plurality of developing sleeves and the plurality of developing sleeves are arranged adjacent to each other.
Further, if the toner scattering occurs at the axial ends of the developing sleeve, photosensitive drum abutting sub-rollers for keeping a distance between the developing sleeve and the photosensitive drum constant will be contaminated by the toner. When the developing apparatus is used for a long term, the toner is accumulated on the abutting sub-rollers, with the result that the distance between the developing sleeve and the photosensitive drum cannot be kept constant (gap is increased), thereby causing reduction in density and/or pitch unevenness. This is particularly unfavorable in a developing system of non-contact developing type in which gap accuracy must be severely controlled, for example, the one-component jumping developing system.
Further, if the toner is scattered at the axial ends of the developing sleeve, the toner will be adhered to the drum or belt (photosensitive member). In this condition, when the image forming apparatus is operated, the toner is fused to ends of the photosensitive member, with the result that poor cleaning may occur in a cleaning portion or contamination at the ends of the photosensitive member mas be accelerated.
In addition, if the toner is scattered at the axial ends of the developing sleeve, the scattered toner will be flying onto a primary charger to contaminate a wire of the primary charger. Particularly, in a low humidity condition, since the charging is unstable, the wire contamination may lead to image unevenness.
An object of the present invention is to provide a developing apparatus having a plurality of developer bearing members, in which scattering and leakage of developer at ends of the developer bearing members can be suppressed.
Another object of the present invention is to provide a developing apparatus in which an image bearing member and other members disposed around a developer bearing member are prevented from being contaminated by toner.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.